The present invention generally relates to the field of noise blanker circuits for selectively blocking the passage of a received signal in response to large magnitude noise impulses. More particularly, the present invention relates to the use of such a noise blanker circuits in a radio receiver.
Noise blanking circuits used in radio receivers generally receive an information containing background noise and noise impulses (abrupt noise spikes) superimposed upon it. The prior art blanking circuits then selectively prevent the passage of the composite signal containing the information signal, background noise and noise impulses by determining when the noise impulses have exceeded some predetermined threshold level. Some prior art noise blanking circuits utilize a controllable gate followed by a holding capacitor, with the controllable gate being controlled in accordance with noise blanking pulses. The blanking pulses selectively pass and block the information signal and superimposed noise in response to noise impulses, with the holding capacitor maintaining the output of the controllable gate constant during the duration of the noise blanking pulses. Some noise blanking circuits also utilize blanking rate shut off circuitry in which the operation of the noise blanker is suspended, or materially reduced, whenever the blanking rate produced in response to rapidly occurring large magnitude noise impulses is so high that continued noise blanking would severely disrupt the quality of the signal passed by the noise blanking pulse controlled gate.
The majority of the prior art noise blanking circuits merely initiate blanking whenever a received noise impulse is detected as having a magnitude exceeding a fixed predetermined threshold level. This is undesirable since there are many times when blanking is desired only for noise impulses which just marginaly exceed the peak noise level of the background noise contained in a received signal. By utilizing a fixed threshold level, the prior art noise blankers either fail to produce any significant blanking for small peak magnitude noise impulses or misinterpret the high background noise level as a series of rapid noise impulses and produce too much blanking thereby degrading the quality of the information signal passed by the controllable gate.
Some prior art blanker circuits have attempted to alter the noise blanking threshold in accordance with the background noise level. However, these blanker circuits, one of which is illustrated in U.S. Pat. No. 4,006,419, have been found not to readily lend themselves to implementation on integrated circuits, and have also been found to be deficient in that the threshold level at which blanking occurs cannot be made to sufficiently closely track the average peak noise level of the background noise contained in an input signal. As a result of this, prior art noise blankers have not been able to initiate blanking for noise impulses which occur at levels slightly above the average peak background noise level because their circuitry did not permit the blanking threshold level to closely track the average peak background noise level as a function of its magnitude variation and in view of expected temperature variations.
In addition, most prior art noise blanking circuits required additional circuitry in order to implement a rate shut off feature which prevented blanking whenever a high repetition rate of impulses noise was encountered. Most of these blankers severely degraded the signal quality of the signal passed by these blankers when such a condition occurred, since the rate shut-off was implemented by sensing when too much blanking had already occurred.